The invention relates to a displacement adjustment device for at least partly converting a rotation into a translation. More in particular, the invertion can relate to a displacement adjustment device, comprising two parts capable of carrying out both a rotation movement and a linear displacement relative to each other, one part being provided with a screw thread, and coupling means being provided for coupling the other part to this screw thread.
Such a device is known in practice for effecting a linear movement by means of a motor. An example of an application of such a device is in a mirror adjusting instrument. When for instance a wing mirror of a vehicle comprises such an instrument, it is easy for the driver to remotely adjust the position of the mirror plate of that wing mirror.
In an elementary embodiment, a known displacement adjustment device has a rotatable gear wheel provided with an internal screw thread, and a bar extending therethrough and provided with an external screw thread which engages with this internal screw thread. The bar is attached to an object to be adjusted, for instance a support plate for a mirror plate of a wing mirror, in such a manner that this bar cannot rotate. When the gear wheel is driven for rotation, the non-rotatable bar is forced to move linearly along its longitudinal axis.
This elementary embodiment has a number of imperfections. In the first place, it may happen that some kind of obstacle restrains the displacement of the bar, such as for intance an end stop. When a driving motor is then energized, it will be unable to rotate, which involves the risk of the motor burning out. In the second place, for some applications it is desired or even necessary that the object to be adjusted can also be adjusted by hand, for instance to enable it to yield to an unanticipated load from outside, but also to permit carrying out an adjustment even in the event of electric failures, which, for reasons of safety, is important in particular with the above-mentioned example of a wing mirror. In the above-mentioned elementary embodiment, such a manual adjustment is not possible.
In order to overcome these imperfections in the elementary embodiment, it is provided with means for enabling a temporary uncoupling. In this connection, the internal screw thread of the rotatable gear wheel is subdivided into a number of segments, which segments are attached to that gear wheel for radial spring action. When an axial force is exerted on those segments, for instance by one of the above-mentioned causes, those segments will be radially forced outwards and thus be released from the engagement with the external screw thread of the bar. Upon further rotation of the gear wheel or further displacement of the bar, the segments will engage with the external screw thread of the bar again, but then shifted by one helix.
Although the elementary embodiment thus modified indeed solves the above-mentioned imperfections, it nevertheless involves some drawbacks.
The first drawback concerns the fact that this construction produces a loud rattling noise, when the object to be displaced reaches an end stop, as well as when the object to be displaced is adjusted manually.
A second drawback concerns the fact that wear occurs, caused by the rattling displacement of the internal screw thread segments over the external screw thread of the bar.
A third drawback, which is in particular involved in the application in a mirror adjusting instrument, is that the object (mirror plate) to be displaced cannot be adjusted continuously (in stepless fashion), but can only reach positions defined by the pitch of the external screw thread of the bar.
A fourth drawback concerns the fact that when the object to be displaced is manually adjusted in a first direction, the required force generally differs from, the force necessary for manual adjustment in the opposite direction.
The object of the invention is to overcome the above-mentioned drawbacks.
German Offenlegungsschrift 2,759,230 describes a displacement adjustment device for pivoting a car mirror by means of a motor. In this known displacement adjustment device, a screw spindle is disposed so as to be rotatable but axially fixed relative to a frame, which screw spindle is driven for rotation by an electromotor with the interposition of a worm/worm wheel transmission. This known displacement adjustment device further comprises an adjusting member which is coupled on the one hand to a mirror and on the other to the screw spindle with the interposition of coupling means. These coupling means comprise a coupling gear wheel which is rotatably mounted relative to the adjusting member and which engages with the screw thread of the screw spindle, the bearing of the coupling gear wheel comprising a slip coupling relative to this adjusting member. The effect achieved with such a construction is that when an end position is reached or when an external force is exerted on the mirror, the parts of the device will not be loaded excessively, because the translation output member can even be axially displaced when the rotation input member is stationary, or the rotation input member can even be rotated when the translation output member is stationary; in those cases, the coupling wheel will rotate. Further, in the case of manual operation, such a displacement adjustment device can be adjusted in a stepless manner.
However, this known construction has some drawbacks. For instance, guide means are required for keeping the coupling wheel in mesh with the screw spindle. For that purpose, the publication describes an intricate construction having a telescopic part which is pressed against the screw spindle by means of spring force. This requires relatively much space, introduces play, and is relatively difficult to assemble. In addition, a user who wants to adjust the mirror manually will have to exert a force whose magnitude is not constant but depends on the pivoting position and the direction of adjustment: in one direction, that spring should be compressed further, whereas in the other direction the spring extends and hence "cooperates". The extent to which the spring should be compressed or extends depends on the angle which the telescopic part makes with the longitudinal direction of the screw spindle.
Another drawback concerns the fact that the screw spindle necessarily extends above the center face of the worm wheel coupled thereto. The lowest position of the axis of rotation of the coupling wheel moving along the screw spindle is located above that center face at a distance from the center face equal to the sum of the radius of that coupling wheel plus half the thickness of that worm wheel. This means that the turn of the coupling wheel in vertical direction is limited, or that the entire construction has a relatively large vertical dimension.